The present invention relates to an inner-focusing high zoom ratio lens, and more particularly, it relates to a downsized light-weighted high zoom ratio lens having barrel components reduced in number.
A prior art inner-focusing high zoom ratio lens has a greater variation in displacement of a focusing lens for focusing to a near point, and because a single focusing cam is insufficient to magnify a satisfactory near-focusing performance for a entire zoom range, a displacement for focusing becomes resultantly increased in the remaining zoom ranges, which leads to a necessity for vari-focal feature. Focusing cam curves used to shift a point of such vari-focal operation to the zoom range require appropriate balance of parameters of zooming and focusing. However, it is difficult to ensure a well-balanced cam configuration to attain smooth zooming and focusing, and hence, a rotation angle of a focusing cam must be altered, or some compensation with the focusing cam is required.
Japanese Patent Laid-Open No. H08-304684 discloses a high zoom ratio lens in which a focusing lens reciprocally moves, forward and backward, while almost simultaneously moving in a circumferential direction during either zooming or focusing. A cam arrangement is used for a guide groove that urges a focusing cam barrel to circumferentially move while a cam pin fitted in the guide groove slides therein along in the focusing cam barrel, so that a displacement in a circumferential direction distorts an operational range with the focusing cam during zooming so as to attain an appropriate adjustment of lens displacement in any of the zoom ranges for focusing.
However, it is still troublesome to ensure a sufficient displacement to magnify a near-focusing performance and to attain a displacement for focusing under a restricted condition of shift of focal point in the entire zoom range.
Additionally, in the high zoom ratio lens disclosed in Japanese Patent Laid-Open No. H08-304684, barrel components are assembled in a telescopic multi-layered barrel structure in which a first cam barrel is disposed inside a fixed barrel while a second cam barrel, a linear barrel, and a third cam barrel are positioned outside the same, and hence, there arises a problem that an outer diameter of the barrel assembly cannot be reduced.
In a high zoom ratio lens disclosed in Japanese Patent Laid-Open No. 2000-89086, a focus compensating cam is provided in a rotational member along with a focusing cam in order to attain an appropriate displacement to focus. However, since a focus lens in such mechanism is designed to linearly move during zooming, a displacement is fixed in any zoom range, and a sufficient displacement for compensation cannot be ensured to shorten a the distance to the nearest focusing point in any zoom range of the high zoom ratio lens.
Also, in the high zoom ratio lens disclosed in Japanese Patent Laid-Open No. 2000-89086, the barrel components are assembled in a five-layer structure where first and second cam barrels, a linear barrel, and a third cam barrel are telescopically deployed outside a fixed barrel, which leads to a problem that an outer diameter of the barrel assembly cannot be reduced.
The present invention is made to overcome the above-mentioned disadvantages in the prior art inner-focusing zoom lens. Accordingly, it is an object of the present invention to provide a high zoom ratio lens that is configured to reduce the number of required barrel components, or namely, the number of required layers of a telescopic barrel assembly, whereby an outer diameter of the barrel assembly can be decreased to make the barrel assembly light-weighted.
It is another object of the present invention to provide a high zoom ratio lens that is configured to reduce a longitudinal dimension of the barrel assembly when it is compacted the most or when a camera is out of use, so that a first one of groups of lenses can be shifted in a relatively greater forward stride.
It is still another object of the present invention to provide a high zoom ratio lens that can further reduce a minimum close-up distance from a front-end lens to an object.
The invention is directed to provide a high zoom ratio lens that is comprised of four groups of lenses, a fixed barrel, a first cam barrel disposed inside the fixed barrel, a linear barrel disposed inside the first cam barrel, a focusing cam disposed inside the linear barrel, and a second cam barrel disposed outside the fixed barrel and rotatably connected to the first cam barrel.
A first one of the groups of lenses are moved for zooming by means of a first cam provided in the second cam barrel, a second one of the groups of lenses serving as a focusing lens are moved for zooming by means of a secondxe2x80x94A cam provided in the linear barrel, and one of third and fourth ones of the groups of lenses are moved for zooming by means of a cam provided in the first cam barrel.
Furthermore, the second group of lenses are moved for focusing by means of a secondxe2x80x94B cam provided in the focusing cam.
The linear barrel is moved for zooming by means of a guide pin provided in the first cam barrel, the third group of lenses are fixed to the linear barrel, and the fourth group of lenses are moved for zooming by means of a fourth cam provided in the first cam barrel. Configured in this way, the high zoom ratio lens of the present invention advantageously has the reduced number of components and resultantly has a simplified arrangement. Both the third and fourth groups of lenses are moved for zooming respectively by means of third and fourth cams provided in the first cam barrel. With this configuration, a freedom in designing optical system is enlarged, and zooming and focusing can be performed with higher accuracies.
The second group of lenses are moved for focusing by engagement of the secondxe2x80x94A cam in the linear cam with the secondxe2x80x94B cam in the focusing cam. With a configuration in this manner, a compensation of cam groove in the second cam can be facilitated.
A displacement of the linear barrel during the zooming is reduced down to 40% to 60% of a displacement of the first group of lenses for zooming. Configured in this way, outer dimensions of the barrel assembly longitudinally compacted the most while a camera is out of use, for example, can be most effectively exploited; that is, the first group of lenses can be shifted to the front end position among others.